I have been building a job board for more than a year now. It makes decent money, more than I expected it to. It's somewhere around 1.000€+ on good months.
On the other hand, it's not quite so passive. I constantly work on it, because I really enjoy working on it. Be it sales, marketing, SEO, building it - love it.
This is also my way of putting the work in to progress my career. I'm learning a lot; experimenting with tools and languages that I want - although I keep it to a bare minimum.